The present invention generally relates to telephone line circuits and, in particular, relates to a feed circuit for such a telephone line circuit employing a floating voltage supply together with the standard central office voltage supply.
Modern telecommunications systems employ digital techniques to implement communications between various subscribers. These systems have become extremely sophisticated and enable the transmission of voice and data in a rapid and reliable manner. An example of a typical system is the ITT 1240 Digital Exchange. This system has been widely publicized and has many innovative features which are the subject matter of many patents. A major consideration in the design of such switching systems resides in the implementation of the line circuit.
In a telephone system the line circuit is responsible for a substantial portion of the cost of the system as each subscriber to the system is associated with a separate line circuit. Hence, workers in the field have been cognizant of such problems and have attempted to simplify the line circuit while making the same more efficient. Thus, the literature is replete with various structures which are dedicated to reducing power dissipation and component count in a line circuit.
See for example, U.S. Pat. No. 4,387,273 entitled SUBSCRIBER LINE INTERFACE CIRCUIT WITH IMPEDANCE SYNTHESIZER issued June 7, 1983 to R. C. W. Chea, Jr. and assigned to International Telephone and Telegraph Corporation, the assignee hereof. See also, U.S. Pat. No. 4,315,106 entitled "APPARATUS FOR REGULATING CURRENT SUPPLIED TO A TELEPHONE LINE SIGNAL OF THE TYPE EMPLOYED IN DIGITAL TELEPHONE SYSTEMS issued on Feb. 9, 1982 to R. C. W. Chea, Jr. and assigned to the assignee hereof.
These patents are but a few of the many patents which relate to line circuits. Essentially, based on the technical strides made in the field of integrated circuit technology, it has been an object of the prior art to provide as many integrated components as possible in order to implement a line circuit structure. It is also known that various functions associated with the line circuit cannot easily be accommodated by the use of integrated circuits. That is, for example, basic line driving functions include the application of a DC voltage to the subscriber line, the accommodation of the high voltage ringing signal, the transmission and reception of voice as well as remote metering signals.
The prior art attempted to implement these functions by the use of various devices, many of which were not capable of being integrated. Recently, there has been a series of developments which enable one to accommodate such functions by the use of wideband amplifier structures. For example, reference is made to a co-pending application entitled A WIDEBAND HIGH VOLTAGE AMPLIFIER FOR TELEPHONE EXCHANGE SUBSCRIBER LINE INTERFACE UTILIZING LOW VOLTAGE CONTROL CIRCUITRY filed on Apr. 10, 1984 as Ser. No. 598,650 for R. C. W. Chea, Jr. and assigned to the assignee hereof.
Therein, a wideband high voltage and high power amplifier that employs a switching converter with the capacitor storage element coupled across a subscriber line is described. The subscriber line is characterized as having a definite load impedance that is mainly resistive and appears in parallel with the storage capacitor of the switching converter. By using the switching converter in such an environment, an extremely efficient, relatively inexpensive, reliable line circuit capable of handling high voltage ringing signals and other signals is achieved.
Another important consideration in the design of a line circuit, since the number of subscribers in a typical telephone system is quite large, i.e., 10,000 or more is the problem of power dissipation. Consequently, even a small savings in power on a per line basis translates into a substantial savings in power from a system viewpoint.